linux-kernel - Re: [PATCH] iommu/arm-smmu-v3: Change vmid alloc strategy from bitmap to ida

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Date:   Tue, 25 Jul 2023 10:41:59 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Dawei Li <set_pte_at@...look.com>
Cc:     will@...nel.org, robin.murphy@....com, joro@...tes.org,
        linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/arm-smmu-v3: Change vmid alloc strategy from
 bitmap to ida

On Sun, Jul 16, 2023 at 12:16:21AM +0800, Dawei Li wrote:
> For current implementation of vmid allocation of arm smmu-v3, a per-smmu
> devide bitmap of 64K bits(8K bytes) is allocated on behalf of possible VMID
> range, which is two pages for some architectures. Besides that, its memory
> consumption is 'static', despite of how many VMIDs are allocated actually.

Is there an actual problem here? Allocating a single 8k page at early
boot doesn't seem like a burden? Are there alot of smmu instances?

> So an IDA based implementation is introduced to address this issue, which
> is capable of self-expanding on the actual need of VMID allocation.

However, I agree IDA is just generally better and generally drivers
shouldn't be open coding it

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason